Computer simulation of alkali metal trapped hole defects in alkaline earth oxides

Alkali metal trapped hole centres are usually unstable species, but there is experimental evidence that spectroscopically identical, stable centres can be formed in alkaline earth oxides. The “microgalaxy” model has been proposed to explain the stability of these centres. We have used static lattice modelling procedures to investigate several features of alkali metal doped alkaline earth oxides and in particular the nature of the impurity-hole interaction. We have calculated solution energies for a range of alkali metal oxides in alkaline earth oxides. The results indicate systems which, thermodynamically at least, are easier to form than those studied experimentally so far. It seems, however, that microgalaxy formation is controlled more by kinetic effects, and preliminary ionic migration calculations bear this out. We have also studied the stabilities of isolated trapped hole defects with respect to the free hole (calculated to be a large polaron) in alkaline earth oxides. These results, and calculated barrier heights to decay of the centre, are in qualitative agreement with experiment. The stabilities of a range of small clusters of alkali metal trapped hole centres were studied and indicated greater thermodynamic stability with increasing size.